Mechanical fluid compressors are used in numerous fields, in many of which, maintenance of high purity levels of the compressed gas or pumped liquid is required. Applications with such requirements include medical applications, such as the provision of compressed gases for respiration support, or for anesthetic use, and cryogenic applications such as in cryo-coolers, where the presence of such contaminants as oil would severely interfere with the operation of the application.
Conventional compressors are classified into rotary and linear motor types. A rotary compressor generally has a shorter lifetime than a linear one due to wear of bearings and the increased piston-cylinder wear caused by radial forces applied by the crank shaft mechanism. Moreover, a rotary compressor produces a troublesome angular momentum, which is hard to eliminate or reduce. In order to increase the lifetime of a rotary compressor, the use of lubricating oil is essential, with its concomitant pollution potential in high purity compression applications. If such rotary compressors are operated without oil, the lifetime of the moving parts would be seriously curtailed. Additional disadvantages of such rotary compressors are heat generation, induced vibrations and noise. In cryogenic applications, the wear products of the moving parts and outgas sing of the lubricants also contaminate the working gas and thus degrade cryocooler performances. On the other hand, linear compressors, though less prone to the negative aspects of rotary compressors, have the disadvantages of lower efficiency, complicated electronic and control systems, and increased weight and volume, particularly because of the electronic drivers required to operate the linear motion generating element.
In the article entitled “A Survey of Micro-Actuator Technologies for Future Spacecraft Missions” by R. G. Gilbertson and J. D. Busch, published in “Journal of The British Interplanetary Society”, Vol. 49, pp. 129138, 1996, a survey is presented of ten different methods applicable to miniature actuators for transforming energy into motion. According to that survey, piezoelectric devices exhibit the highest efficiency, fastest speed of operation and highest power density relative to other methods. These advantages make piezoelectric devices potentially attractive for implementation in miniature gas compressors. Furthermore, the lack of rotating parts increases their reliability compared with conventional rotary compressors, this being an important feature in medical uses, and in military uses, such as in cryocooler compressors for low-temperature infrared detectors.
The major problem in employing piezoelectric elements as compressor actuators is the extremely small elongation of the piezo materials, typically about 0.1% of the total actuator length, and thus of the order of microns in standard piezo actuators, such as those of Lead Zirconate Titanate (PZT), which is probably the most widely used piezoelectric material, and which will be used as the example material in this disclosure. Such small strokes create technological problems to implement, associated with the dimensional and geometry tolerances, surface finishing, structure stiffness and more. Another significant disadvantage of the PZT actuators is the low power density and electromechanical efficiency achievable from piezoelectric elements when operated at the “low” frequencies required for practical compressor operation, which are typically in the range of a few tens to a few hundred Hz. For instant a Stirling-type cryocooler based on piezoelectric elements should operate in the frequency range of 50-150 Hz. However, direct quasistatic wave generation using piezoelectric actuators at such low frequencies is extremely inefficient. At these frequencies, about 90% of the PZT charge is wasted, mostly because of the elasticity of the PZT ceramic itself. To improve the efficiency of a piezoelectric compressor, it is essential to operate the PZT element at its mechanical resonance, and since the natural frequency of PZT stack actuators is generally of the order of tens of kHz, a mechanism must be found for reducing the resonant frequency by about two orders of magnitude.
High frequency piezoelectric compressors incorporating a frequency reduction mechanism with a complex hydraulic transmission system have been reported. However, even in such systems, the piezoelectric element cannot be operated at a frequency as high as its natural resonance, due to frequency limitations of the check valves used in the hydraulic transmission system and the high hydraulic losses at such frequencies.
Some of the problems arising from piezoelectric/hydraulic systems have been considered in a number of prior art publications, including in International Patent Application published as WO 2009/010971 for “Piezo-Hydraulic Compressor/Pressure Oscillator for Cryogenic Cooling and other Applications” to the applicant of the present application; the article entitled “Performance Modeling of a Piezohydraulic Actuator with Active Valves, by H. Tan et al., in Smart Materials and Structures, Vol. 14, pp. 91-110 (2005) published by IOP Publishing of Bristol, U.K.; in the article entitled “Investigation of the Dynamic Characteristics of a Piezohydraulic Actuator” by J. Sirohi et al., in “Journal of Intelligent Material Systems and Structures”, Vol. 16, pp. 481-492 (June 2005), published by Sage Publications of London EC1, UK; and in references cited in those various publications.
There therefore exists a need for a linear piezoelectric compressor which overcomes at least some of the disadvantages of prior art systems and methods.
The disclosures of each of the publications mentioned in this section and in other sections of the specification, are hereby incorporated by reference, each in its entirety.